The present invention generally relates to processes for fabricating silicon-on-insulator wafers, and more particularly, to a technology for thinning of a semiconductor substrate by separating a layer from an initial substrate with a hydrogen interlayer.
In previous art, a process for lift-off of a thin layer from a substrate has been described by Gmitter at al. [1]. This process uses etching of sacrificial layer. A disadvantage of the process is that the layer that can be lifted off is limited to about 1 cm2 in area. Fabrication of silicon-on-insulator wafers, however, currently requires lifting off layers with an area of 100 to 1000 cm2.
Another process known from previous art is thinning of a substrate by etching away the substrate until an etch stop layer is reached. For example, in the invention by Yonehara [2] a porous silicon layer inside of crystalline silicon wafer is used as the etch-stop layer. A disadvantage of etch-stop based processes is that an entire semiconductor wafer is thus sacrificed and cost is thus increased.
Yet another process for separating of semiconductor substrate is known due to invention by Bruel [3]. In this process a gaseous interlayer is formed inside of semiconductor wafer by the sequence of process steps (1) implantation of hydrogen, (2) stiffening the surface that was implanted through to prevent blistering, and (3) transforming of the implanted hydrogen into quasi-continuous hydrogen layer. A disadvantage of the process is that a high implant dose is needed (1017/cm2 for monatomic hydrogen or 5xc3x971016/cm2 for diatomic hydrogen). The total cost of the SOI-end-product wafer using this process is increased by the cost of the implantation.
A slight improvement to the previous process is known due to invention by Goesele [4]. Due to Goesele""s process a two-species implantation (boron and hydrogen) is used that allows lowering the total dose needed by about 20%. Boron is implanted first and it works as a trap-inducing step to reduce the density of hydrogen implant required. Under subsequent annealing the implanted hydrogen is fully gettered by the trap layer, thus lowering the hydrogen losses through effusion. All implanted hydrogen is thus used to build hydrogen platelets that are useful for the lift-off process. A disadvantage of this process is that the implantation dose (and related cost) remains high. Another disadvantage of Goesele""s process is that hydrogen implantation produces defects and the as-implanted wafer contains the hydrogen in a trapped form. In the subsequent annealing process Goesele teaches that the implanted hydrogen releases from its attachments to hydrogen implantation-induced defects, and attaches to boron implant-induced defects. Releasing from attachments requires annealing out of defects. Both hydrogen- and boron- implantation-induced defects have similar microscopic nature, and same annealing temperatures. Therefore when hydrogen is released, the boron-induced traps are mostly annealed out, and the hydrogen re-attachment process is not very effective. That is why only a 20% improvement in total implant dose was obtained by Goesele.
Another improvement of Bruer s process is described in the invention by Henley at al. [5]. This process uses plasma immersion ion implantation instead of conventional implantation to insert hydrogen into silicon. A disadvantage of the process is that plasma immersion implantation results in an energy distribution of incident hydrogen over a wide energy range to about 50-80 keV. This results in a lo times increase of minimum implant dose needed for cleavage (i.e. 1018 cm2 instead of 1017 cm2). This high dose damages the layer to be lifted off, and the quality of the final SOI wafer is lower as compared to wafers obtained using Bruel""s process. Another problem of plasma immersion is that the high dose implantation can result in severe damage to the wafer surface. This surface should be immediately attached (without any intermediate heat treatment to heal the surface) after implantation to a stiffener wafer. Prebonding of the damaged surface to another surface is not effective. Thus plasma immersion lowers the production yield of silicon-on-insulator wafers significantly.
Besides implantation, another process is known to insert the hydrogen into silicon and is described in papers published by de Mierry at al. [6], Pearton at al. [7], Oehrlein at al. [8], Raghavan at al. [9]. The process uses electrolytic insertion of the monatomic hydrogen into silicon. Monatomic hydrogen diffuses inside crystalline silicon at room temperature thus allowing easy doping of silicon with hydrogen. However, electrolytic insertion cannot be directly applied by itself alone to achieve a substrate split. The present invention describes a process utilizing electrolytic insertion to achieve lift-off of surface films.
The present invention relates to a method for lifting off of a thin layer from a crystalline substrate. The fractured layer may be further attached to another substrate thus forming, for example a silicon-on-insulator (SOI) wafer.
In a first step in the inventive process a hydrogen trap-inducing implantation of an element is performed where said element is preferably the same as the substrate material (for example, silicon in case of a silicon wafer). This described self-implantation has two major advantages for this process (a) it does not contaminate the substrate, and (b) bombarding a target with particles having the same mass as atoms of target allows the most effective energy transfer from bombarding particle to an atom of the target thus maximizing atomic displacements. The purpose of said implantation is to obtain a buried amorphous layer. The amorphized layer is a trap for monatomic hydrogen inserted at the next step of the inventive process.
In a second step a hydrogen insertion by electrolytic means is carried out which supplies hydrogen in an amount that is enough to subsequently form a quasi-continuous gaseous interlayer in the region of amorphization.
In a third step the substrate is subjected to a sensitizing heat-treatment at an elevated temperature for a given time. Said heat treatment is chosen such that the hydrogen in the substrate which was introduced by the hydrogen electrolytic insertion and trapped by the buried amorphized layer does nucleate into platelets. This treatment causes the formation and growth of hydrogen filled platelets at a depth close to the depth of the buried amorphized layer. Said heat treatment must be maintained below a temperature which causes hydrogen induced surface blisters which in turn would prevent subsequent effective bonding of the substrate to a stiffener wafer.
In a fourth step an intimate and strong bond between the substrate and the stiffener is realized by direct wafer bonding or other bonding techniques.
In a fifth step the bonded structure consisting of the composite substrate and stiffener is heat-treated at a transfer temperature whereby the implanted hydrogen in the substrate is fully released from defects which were generated by the hydrogen trap-inducing amorphization. Said heat treatment results in growth, overlapping and coalescence of hydrogen-filled platelets. Said platelets split the monocrystalline thin layer from the rest of the substrate thereby transferring the thin monocrystalline layer to the stiffener which completes the lift-off process.